Soluble & Insoluble Fiber: What is the Difference?

QUESTION:

What's the difference between soluble and insoluble fiber? Are they both good for you, and how can I include them in my diet?

ANSWER:

Dietary fiber, the edible portions of plant cell walls that are resistant to digestion, is an extremely beneficial component of our diets. Not only does it help ward off many diseases, it has been shown to aid in weight loss by reducing food intake at meals. This is because fiber-rich foods take longer to digest and thus result in an increased feeling of fullness and satiety. In addition, the more gradual absorption slows the entrance of glucose into the blood stream, thereby preventing large blood glucose and insulin spikes.

The recommended fiber intake is 20 - 35 grams per day for adults, or 10 - 13 grams for every 1,000 calories in the diet. This recommended amount should come from a combination of soluble and insoluble fiber, since each type provides different benefits. While it's not necessary to track, a 3:1 ratio of insoluble to soluble fiber is typical. Although neither type is absorbed by the body, they have different properties when mixed with water, hence the designation between the two. However, due to overlap in function between the two types and disparities in measurements of each depending on the method used, the National Academy of Sciences has recommended that these terms "gradually be eliminated and replaced by specific beneficial physiological effects of a fiber". Thus you may hear less about "soluble vs. insoluble fiber" in the future.

Soluble fiber is "soluble" in water. When mixed with water it forms a gel-like substance and swells. Soluble fiber has many benefits, including moderating blood glucose levels and lowering cholesterol. The scientific names for soluble fibers include pectins, gums, mucilages, and some hemicelluloses. Good sources of soluble fiber include oats and oatmeal, legumes (peas, beans, lentils), barley, fruits and vegetables (especially oranges, apples and carrots).

Insoluble fiber does not absorb or dissolve in water. It passes through our digestive system in close to its original form. Insoluble fiber offers many benefits to intestinal health, including a reduction in the risk and occurrence of hemorrhoids and constipation. The scientific names for insoluble fibers include cellulose, lignins, and also some other hemicelluloses. Most of insoluble fibers come from the bran layers of cereal grains.

Since dietary fiber is found only in plant products (i.e., nuts, whole grains, legumes, fruits and vegetables), these are essential to a healthy diet. The average American significantly falls short of the recommended amount of fiber, consuming on average only 12 - 17 grams per day. Ways to increase dietary fiber in your diet are:

Replace meat with beans or other legumes in meals. Lentils are perfect for this!

Try experimenting with the above tips. Slowly modify recipes until you attain a balance that is appetizing and tasteful to your taste buds. If you are not accustomed to a high-fiber diet, increasing fiber intake slowly will minimize any gas or bloating.

Our expert, Dr. Sharon E. Griffin, holds a B.S., M.S., and Ph.D. in the areas of exercise science/physiology. She also holds a second M.S. degree in Nutrition and is a licensed nutritionist and an ACSM certified health and fitness instructor.